In recent years, there has been a rapid advance toward finer pattern rules for high integration and high speed performance of LSI devices. The application of shorter-wavelength exposure light sources is seen as one factor behind the advance toward finer pattern rules. For example, the wavelength reduction from a mercury lamp i line (365 nm) to a KrF excimer laser (248 nm) enables mass production of 64-Mbit DRAM (Dynamic Random Access Memory) (with a processing size of 0.25 μm or smaller). The application of lithography process using an ArF excimer laser (193 nm) has also been thoroughly studied to produce DRAM with an integration of 256M and 1G or higher. In particular, the combination of the ArF lithography process with a high NA lens (NA≧0.9) is being studied for production of 65-nm node devices. For production of next 45-nm node devices, a F2 laser of 157 nm wavelength is considered as a candidate for use in lithography process. However, the application of the F2 lithography process has been postponed due to many problems such as increase in scanner cost, change of optical system and low resist etch resistance. As an alternative to the F2 lithography process, ArF immersion lithography process has been proposed. Development is currently proceeding for early introduction of the ArF immersion lithography process.
As resists suitable for use in such an exposure wavelength range, attention is being given to “chemically amplified resist materials”. The chemically amplified resist material is a pattern forming material that contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”), which is capable of generating an acid by irradiation of a radiation (hereinafter referred to as “exposure”), and forms a pattern by making a difference in developer solubility between exposed and unexposed portions through a reaction using the acid generated by exposure as a catalyst.
Various researches have been made on photoacid generators for use in chemically amplified resist materials. Conventionally, a chemically amplified resist material for exposure to a KrF excimer laser uses a photoacid generator that generates an alkane- or arene-sulfonic acid. It is however known that the use of such a photoacid generator in an ArF chemically amplified resist material does not cause a sufficient acidity for cleavage of an acid labile group of the resist resin and thereby results in resolution failure or low resist sensitivity so that the resist material cannot be suitable for device production.
For this reason, the ArF chemically amplified resist material generally uses a photoacid generator that generates a perfluoroalkanesulfonic acid of high acidity such as a perfluorooctanesulfonic acid, or a derivative thereof, known by its acronym “PFOS”. There have however been discussed problems about the stability (non-degradability) of PFOS due to C—F bonds as well as the biological concentration and accumulation of PFOS due to hydrophobic and lipophilic natures. The above problems start being raised to perfluoroalkanesulfonic acids having a carbon number of 5 or more.
In order to cope with these PFOS-related problems, the development of partially fluorinated alkanesulfonic acids of lower fluorine substitution degree has been pursued by manufacturers. For example, Patent Document 1 describes the development of α,α-difluoroalkanesulfonic acid salt from α,α-difluoroalkene and a sulfur compound and discloses a resist material containing such a sulfonic acid salt, e.g., di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate, as a photoacid generator to generate a corresponding sulfonic acid by exposure. Patent Document 2 describes the development of α,α,β,β-tetrafluoroalkanesulfonic acid salt from α,α,β,β-tetrafluoro-α-iodoalkane and a sulfur compound and a photoacid generator using this sulfonic acid salt as a photoacid generator to generate a corresponding sulfonic acid and a resist composition containing the photoacid generator. Patent Document 3 discloses a photoacid generator having a difluorosulfoacetic acid alkyl ester (e.g. 1-(alkoxycarbonyl)-1,1-difluoromethanesulfonate), a difluorosulfoacetic acid amide (e.g. 1-carbamoyl-1,1-difluoromethanesulfonate) or the like although there is no mention made on the synthesis method of this photoacid generator. Further, Patent Document 4 discloses triphenylsulfonium (adamantan-1-ylmethyl)oxycarbonyldifluoromethanesulfonate although there is no mention made on the synthesis example of this sulfonate compound. Patent Document 5 discloses a triphenylsulfonium alkyloxycarbonyldifluoromethanesulfonate having a lactone structure etc. Furthermore, Patent Document 6 discloses triphenylsulfonium 2-acyloxy-1,1,3,3,3-hexafluoropropanesulfonate etc.
Patent Document 1: Japanese Patent Application Publication No. 2004-531749
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-2252
Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-214774
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-4561
Patent Document 5: Japanese Laid-Open Patent Publication No. 2006-306856
Patent Document 6: European Patent Application Publication No. 1710230A1